How to adjust normal map
Actually yes. It took me a while. Normal mapping is going to be one of the hardest things to do right. With a multi object, model you can explode the mesh and use displacement modifier cages. I don't think this model can do that though, not easily anyways, its a smooth organic and the cage might need a lot of tweaking..

I dont know if you could rig and pose the model in order to create a cage for the small areas like the toes armpits and groin. If you used a displace modifier to create a cage those sinchy areas would be toughest. You might have to fineagal the cage mesh for a better bake. I could be very wrong here, im still a novice. From what I've seen you may need to manually paint those problem areas in the normal map in GIMP. I wouldnt even know where to begin with that. Characters are kind of hard. If this was an object you could split it into parts and bake easily.

Please don't throw me in an oven if someone more experienced blows my suggestions out of the water.

On udemy theres a 3dcoat and blender tutorial on making a sword. It does an amazing job describing the normal map bake process.

https://www.udemy.com/intermediate-game-asset-creation-3d-modeling-in-blender/learn/v4/overview